Beverage containers, and the like are commonly made by blow molding a parison, or preform, that is made from polyethylene teraphthalate (PET) material. Such PET reheat and blow preforms are commonly manufactured in an injection molding machine. Injection molding machines are typically equipped with a hopper that contains particulate thermoplastic polymer resin, usually in pellet form. The resin particles are fed to an extruder where they melt under the application of thermal and shear energy. The resulting molten resin is then fed to an injection nozzle and injected into a mold. Once the molded resin has set, or frozen, the resulting plastic article is ejected from the mold, and the process repeats.
Extruders, also called plasticizers, are well known to those of skill in the field of injection molding. Generally, the hopper feeds the inlet end of a plasticizing screw which is encased in a heated barrel of constant diameter. Helical flights on the screw convey the particles along the screw where the mass of particles are compressed and rub against the heated barrel surface. This action provides frictional heating as well as some heat conducted from the barrel. However, the bulk of the energy imparted by the screw to the particles is the result of shear. The particles are sheared between two surfaces, the root diameter of the screw and the inner diameter of barrel, which move with respect to each other. The heat generated by this shearing action increases along the length of the barrel as the screw root increases in diameter closer to the outlet end of the screw, so that a homogeneous melted mass of the plastic polymer material is produced.
In a reciprocating screw type of injection molding machine, the polymer material accrues in the space beyond the discharge end of the screw. When a predetermined amount of material has accumulated, the screw stops its rotation. It then serves as a plunger, moving forward and forcing a desired quantity of the polymer melt, or a "shot", through the injection nozzle and into a mold or molds with one or more cavities. The polymer melt cools in these cavities and the finished molded articles are ejected from the mold or molds.
In a two stage or pre-plasticizing injection system the role of the plasticizing or extruder screw is restricted to melting the material, with the injection of the molten plastic into the injection mold accomplished by a separate shooting pot and injection plunger or piston system.
Prior art extruders have a number of well known disadvantages. The design of extruder screws is a complex art. The ratio of length to diameter of the screw and barrel, and the speed with which the screw can be rotated, are restricted by a number of factors, including plasticizing rates, residence time in the extruder between shots, and shear sensitivity of the particular polymer material. This can limit the throughput of the extruder and prevent significant reductions in cycle time for the injection molding process. Further, increasing the speed of transfer of the melt usually exposes the melt to high shear rates due to resistance to the flow of the melt. Excessive and uncontrollable shear rates, and their related heating effect on the melt contribute to the formation of degradation products. The predominant degradation product generated during standard processing of PET is acetaldehyde. Its presence in molded objects such as beverage bottles, water bottles and food containers and the like is very deleterious from a taste standpoint, even at very low concentration levels. Consequently, an important focus of optimization in the plasticizing screw is the reduction of polymer degradation while maintaining melt output. In addition, control of heating in screw plasticizers is problematic. Generally, zone heating is provided along the barrel, but this is a somewhat crude technique and even heating throughout the melt is difficult to achieve.
From a physical standpoint, the size of the extruder is often the main determining factor in the size of an injection molding machine. While it is clearly desirable from an industry perspective to reduce the footprint of injection molding machines, the length of barrel required to fully plasticize the polymer material translates into increased floor space requirements. Furthermore, barrel maintenance and change is a slow and labourious process with the prior art extruders.
U.S. Pat. No. 5,200,204 describes a rotary plasticizing system that dispenses with a screw. Instead, the particulate polymer material is fed into an annular space between a rotor and a barrel. Eccentric rotation of the rotor within the barrel imparts shear energy to the polymer material causing the material to melt. Heating can also be provided along the length of the barrel. While this rotary extruder is more compact than the typical screw extruder, its relatively low throughput, and the difficulties in adapting it to production molding applications, has hampered its adoption in the injection molding industry.
Extruders are also employed in related molding fields. For example, thixotropic materials, such as magnesium, melted in an extruder and fed into a suitable mold. As used herein, plasticization is defined to include the transformation of any material from a solid state to a flowable, or moldable, state under the application of thermal and/or shear energy, and is not limited to the plasticization and/or extrusion of polymer plastic resins.
As is well known in the unrelated heating and refrigeration field, compressors are a common component used to compress the refrigerant fluid to extract heat from the system. Compressors are usually either reciprocating compressors that use a reciprocating piston to compress the fluid, or scroll compressors. Generally, scroll compressors employ two interfitted involute scroll members. One of the scroll members is stationary, while the other orbits eccentrically about the stationary member. The refrigerant is introduced into a pocket formed at the outer edge between the two scroll members. Provided the two scroll members are restrained from rotating relative to each other, as the orbiting scroll traces its path, the refrigerant is moved to the center of the enmeshed scroll members, while being simultaneously compressed as the size of the pocket is continuously reduced. Though well known for nearly a century, the elegant scroll compressor has been limited to refrigeration and gas compression applications using compressible fluids such as freon.
It is, therefore, desirable to provide a novel extruder for plasticizing and extruding material that can occupy less space, and provide improved control of melt characteristics, such as evenness of heating through the melt.